The term faujasites is used here to refer to a group of mineral species characterized by their crystallographic topographic structure, an excellent account of which can be found in the work by Donald W. Breck "Zeolite Molecular Sieves," published by John Wiley and Sons, 1974 pp. 92 ff. The so-called Lowenstein rules requires that they have an Si/Al ratio which is high, or at least equal to 1. Customarily the following distinctions are made:
X faujasites with Si/Al&lt;1.5, PA1 Y faujasites with Si/Al&gt;1.5.
The classical X faujasites have an Si/Al ratio R1x.gtoreq..1.2. The physical characteristics of the faujasites (exchange capacity, nitrogen adsorption, etc.) evolve as a function of the Si/Al ratio. The topic here relates to the faujasites with a Si/Al ratio equal to 1, which are called the LSX faujasites (in English Law Silica X faujasites), because of all the faujasites which can be considered, those are the ones which possess the maximum possible number of tetrahedral aluminum ions per crystalline unit, and because as a result they potentially have the highest number of adsorption sites for a very great variety of molecules such as CO.sub.2, N.sub.2, etc. It should be specified that in the sense of the present invention, LSX faujasite is understood to represent a zeolitic species whose Si/Al ratio is measured as 1.+-.0.05, where values below 1 reflect the analytical uncertainties in the measurement of this ratio, and higher values reflect either the same analytical uncertainty or a tolerable discrepancy in the purity of the product.
The process of synthesis of faujasites consists of two distinct steps, the first (aging) in which the precursor seed crystals appear, the second (crystallization) is a phase of growth of crystals from these seeds. The course of these steps depends on numerous physical and chemical factors which are not always well understood. In any case, the conditions of the medium determine the type of seeds formed and consequently the type of zeolite. What is known in any case is that the difficulties of the synthesis of faujasites increase when their Si/Al ratio decreases, and that the traditional methods fail for the preparation on a large scale of faujasites with an Si/Al ratio of 1, which are the object of the present invention. In particular, from a pure sodium medium the zeolite NaA is obtained, and it is necessary to introduce a certain quantity of potassium ions to synthesize the faujasite structure; it is recognized that the faujasite structure can be obtained for Na/Na+K ratios in the synthesis gel of 0.7-0.8. The synthesis of such zeolites with low Si/Al ratio has been described in the French Patent No. 2,357,482.
Very generally, the zeolites are prepared by crystallization of silicoaluminum aqueous compositions formed by a mixture of solutions of alkaline aluminates and sodium silicate at a temperature between room temperature and a temperature below their boiling point. With regard to the stirring conditions in these media, one is torn between two contradictory imperatives. Stirring means favoring the exchanges of matter and heat, and preventing the syneresis of the forming crystals. The other option is not to stir; in fact it has been observed in the synthesis of faujasites that stirring systematically led to certain disturbances, particularly a defect in the crystallinity of the zeolite, which was reported by R. M. Barrer (see Hydrothermal Chemistry of Zeolites, Academic Press, 1982, p. 171). It can also be observed, independently of an insufficiency of crystallinity, that parasitic phases appear; these phases form as a result of the destruction due to the stirring of certain unstable seeds. It is thought that in these cases, the local shearing can promote the appearance of certain seeds and the disappearance of certain others (see, for example, D. E. W. Vaughan, Chem. Eng. Prog., 48(2), 1988, pp. 25-31). In practice, for the industrial manufacture of LSX, inspiration is obtained from the teaching of G. H. Kuhl (Zeolites, September, 1987, Vol. 7, pp. 451-457) and one omits the stirring, resigning oneself to longer aging and crystallization durations and thus to a very unsatisfactory industrial profitability.